1. Field of the Invention
The invention generally relates to adhesive compositions and, more specifically, to extrudable adhesive compositions comprised of grafted and ungrafted polyolefin components and an adhesion promoter.
2. Description of the Prior Art
Adhesive blends, commonly referred to as tie layer adhesives, to improve adhesion between polyolefins and dissimilar substrates, such as polyamides and ethylene-vinyl alcohol (EVOH) copolymers, metals and the like, in multi-layer constructions are well known. These adhesive blends typically consist of a base resin, which is the predominant component in the blend, and a modified polymer containing carboxylic or anhydride functionality, e.g., polymers grafted with maleic anhydride. Optionally, one or more other polymeric materials, including rubbers, may be included in the adhesive blends. Representative adhesive blends of the above types are described in U.S. Pat. Nos. 4,087,587; 4,298,712; 4,487,885; 4,774,144; and 5367022.
While numerous tie-layer adhesive compositions have been developed, new formulations which extend the envelope of performance or meet the requirements of new laminate applications are always in demand. Tie-layer adhesives having improved adhesion, particularly at high temperatures, are especially desirable. High temperature conditions may occur as a result of the processing or manufacturing operations or the environment to which the finished article is exposed. Examples of the latter would be for multi-layer tubing for under-the-hood automotive applications where the lines can be exposed to high temperatures for extended periods and where adhesive compositions resistant to thermal degradation are therefore essential. Other applications where heat resistant tie-layer adhesives are necessary are for heat shrinkable multi-layer films used for cook-in applications. Still another application requiring good high temperature performance is for tubing for plumbing applications where the tubing can also undergo repeated heating and cooling cycles and the associated stresses resulting therefrom.
Composite polyolefin/metal pipe and tubing is extensively used for plumbing and heating systems. Polyethylene (PE) and crosslinked PE (PEX) and aluminum are most commonly used for these applications. These constructions typically would have the structure PEX/tie-layer/metal/tie-layer/PEX or PE/tie-layer/metal/tie-layer/PE. Even though the metal provides most of the mechanical strength in these constructions, mechanical integrity and pressure ratings are dependent on the quality of the bond between the metal and PE or PEX. It would be highly desirable if adhesive compositions having improved metal adhesion were available for these applications.
Adhesive compositions having improved metal adhesion particularly at elevated temperatures are now provided. The compositions consist of blends of a polyolefin base resin and graft modified polyolefin to which a low level of adhesion promoter is added. The incorporation of the adhesion promoter significantly increases adhesion even at temperatures as high as 120xc2x0 C.
More specifically the improved adhesive compositions of the invention are comprised of (a) an adhesive blend of 75 to 97.5 wt. % polyolefin base resin having a melt index of 0.1 to 100 g/10 min. and 2.5 to 75 wt. % modified polyolefin grafted with 0.5 to 4 wt. % ethylenically unsaturated carboxylic acid or acid derivative; and (b) 50 to 10000 ppm, based on the adhesive blend, of an adhesion promoter selected from the group consisting of organic bases, carboxylic acids and organometallic compounds.
Particularly useful compositions are obtained when the base resin is selected from the group consisting of ethylene homopolymers; copolymers of ethylene with C3-8 xcex1-olefins, vinyl carboxylates and alkyl acrylates; terpolymers of ethylene and propylene with diene monomers; polyisobutylene; and copolymers of isobutylene and isoprene; the graft modified polyolefin is polyethylene grafted with maleic anhydride; and the adhesion promoter is a carboxylate of a metal selected from the group consisting of lead, cobalt, iron, nickel, zinc and tin. Dibutyltin dilaurate is an especially useful adhesion promoter.
In another highly useful embodiment, the base resin is a mixture of a highly crystalline polyolefin resin and elastomeric polyolefin resin present at a weight ratio of from 15:1 to 1:1. Base resins comprised of mixtures of polyethylene resins with EPR, EPDM or polyisobutylene provide particularly good adhesion especially when employed with maleic anhydride grafts of HDPE or LLDPE.